1. Field of the Invention
The present invention relates to an optical fiber, an optical fiber preform (i.e., base material), and a manufacturing method therefor, and in particular, relates to an optical fiber whose refractive index distribution is controlled so as to perform high-quality and high-speed transmission, a relevant optical fiber preform, and manufacturing methods therefor.
2. Description of the Related Art
In the field of optical communication, optical fibers having a refractive index distribution having a rectangular or triangular shape are mainly designed. Examples of design values for such a refractive index distribution are shown in the graphs of FIGS. 5 and 6. In each graph, the horizontal axis indicates the position in the radial direction of the optical fiber, where the outer diameter of the cladding is defined as 100%, and the position as percentage from the center (0%) to the outer periphery (50%) of the cladding is shown (i.e., the left half (0 to −50%) is omitted in the graph). On the other hand, the vertical axis shows the relative refractive index with respect to the refractive index of the cladding.
In these examples, values for the refractive index distribution are symmetrical with respect to the center of the core of the optical fiber. Therefore, in FIGS. 5 and 6, the left end of each graph corresponds to the center of the core, and the right end of the graph corresponds to the periphery of the cladding. Accordingly, FIG. 5 shows a rectangular-shaped refractive index distribution of an optical fiber, in which the refractive index is constant in the core, and FIG. 6 shows a triangular-shaped refractive index distribution of an optical fiber, in which the refractive index is maximized at the center of the core and then decreases at a fixed rate in the core. The optical fibers having the above-explained refractive index distributions are generally used because the optical characteristics of these optical fibers can be easily estimated based on their refractive index distributions.
That is, the optical fiber has a core having a higher refractive index and a cladding having a lower refractive index. In order to obtain such a refractive index distribution, the main component of the core and cladding of the optical fiber is highly pure silica glass (SiO2), and a part or all of the entire material is doped with a dopant for increasing the refractive index or a dopant for decreasing the refractive index.
The dopant for increasing the refractive index may be GeO2, TiO2, SnO2, ZrO2, Nb2O5, Ta2O5, Yb2O3, La2O3, Al2O3, or the like. The dopant for decreasing refractive index may be B2O3, F, or the like. Additionally, in order to improve the optical characteristics of glass (i.e., SiO2) such as the softening point, the coefficient of thermal expansion, the chemical resistance, the transition point, and the dispersion loss, SiO2 is doped with a dopant as explained above or P2O5, and further doped with another known dopant if necessary. As a more concrete example relating to the composition of the optical fiber, the core is made of SiO2 which is doped with GeO2 and the cladding is made of SiO2.
However, generally, the refractive index distribution of the actually manufactured optical fiber has values different from the design values as shown in FIG. 5 or 6. In particular, when the optical fiber is manufactured using a VAD (vapor-phase axial deposition) method, the portion corresponding to the core is produced in a single process; thus, the probability of producing an optical fiber whose refractive index distribution has values different from the design values is high.
In addition, in the conventional process (including a heating process) of manufacturing the optical fiber, the concentration of the dopant for controlling the refractive index, which should be added only to the core area, is inevitably irregular, and such an additive (i.e., dopant) is also inevitably diffused towards the cladding area.
Therefore, irregular concentration of the additive tends to be produced in the vicinity of the boundary between the core and the cladding. Such irregular concentration of the additive produces a portion where the refractive index steeply changes (called the “refractive-index steep change portion” hereinbelow).
In conventional design of the refractive index profile, the presence of such a refractive-index steep change portion has not been considered. However, in the actually manufactured optical fiber, such a steep change in the refractive index affects the optical characteristics of the fiber, in particular, the wavelength dispersion, so that the wavelength dispersion has a value different from that anticipated in design. Such an error in the wavelength dispersion causes a waveform distortion in the optical transmission, thereby affecting the high-quality and high-speed transmission.
On the other hand, when the optical fiber is manufactured by the VAD method, (i) the refractive index distribution of a manufactured optical fiber preform is measured and the amount of drawing of the optical fiber preform is determined based on the measurement results, (ii) the refractive index distribution of the optical fiber preform after the drawing is again measured and the amount of the outside deposition is determined based on the measurement results, (iii) the refractive index distribution of the optical fiber preform after the outside deposition is again measured for confirmation, and (iv) drawing of the optical fiber preform as produced above is performed so as to produce an optical fiber.
Here, if the optical fiber preform includes a refractive-index steep change portion as explained above, the above measurement of the refractive index distribution cannot be accurately performed, so that it is difficult to produce an optical fiber having target characteristics.